The question "Why is AC current smaller than DC current?" is misleading because it is not necessarily true that AC current is always smaller than DC current. Instead, a more accurate question would be: Why is AC voltage often higher than DC voltage in power transmission, resulting in lower current for the same power delivery? This is because the amount of current is determined by the voltage applied, and the resistance of the circuit through Ohm's Law (Current = Voltage/Resistance), and the amount of power to transmit. It's the transmission of AC power at high voltages that leads to lower currents in transmission lines.
Understanding AC and DC
Before we delve into why AC current is often lower in power transmission, let's briefly differentiate between AC and DC:
- Direct Current (DC): Flows in one direction only. Batteries and electronic devices typically use DC.
- Alternating Current (AC): Periodically changes direction. The electricity in our homes is AC.
The Importance of High-Voltage AC Transmission
The primary reason AC is favored for long-distance power transmission relates to efficiency and cost:
| Property | AC | DC |
|---|---|---|
| Direction of flow | Changes periodically | One Direction |
| Long distance transmission efficiency | High | Can be high but more complex and costly |
| Voltage Conversion | Easy with transformers | More complex and expensive |
| Current during Transmission | Lower for same power | Higher for same power |
Here's why high voltage (and thus lower currents) is desirable for transmitting electricity over long distances:
- Reduced Energy Loss: As stated in the provided reference, "At high voltages (over 110kV), less energy is lost in electrical power transmission." When electricity travels through a wire, some energy is lost as heat due to the resistance of the wire. This heat loss is proportional to the square of the current (I2R). Therefore, transmitting the same amount of power at a higher voltage allows for lower current, drastically reducing heat loss and energy waste.
- Example: If you double the voltage while keeping the same power output, you can halve the current. This reduces the heat loss to one-quarter ( (1/2)^2 = 1/4) of what it would have been.
- Transformers: One key advantage of AC is that its voltage can be easily increased or decreased using transformers. This ability is crucial for efficient long-distance transmission (high voltage for minimal loss) and safe distribution to homes and businesses (lower voltage). DC voltage conversion is more complex and expensive.
- Lower Infrastructure Cost: Due to reduced heat loss, power companies can use thinner, less expensive transmission wires, since they don't need to accommodate the excessive heat generated by higher currents.
Practical Insights
- Household Power: While the electricity reaching our homes is AC at a relatively lower voltage (e.g., 120V in the US or 230V in Europe), power companies initially transmit this electricity across large distances at much higher voltages, in some cases hundreds of kilovolts (kV).
- Step-Down Process: Transformers are used along the way to "step down" this high voltage to safer and usable levels.
- DC Transmission: Although AC is generally preferred for long-distance transmission, DC transmission (HVDC - High Voltage Direct Current) systems are used in some situations where they offer advantages, such as very long underwater cables. This is because the current is constant in DC lines, and AC lines have a higher reactive effect, so it is more efficient at very long distances.
Conclusion
The reason that AC is often associated with lower currents in power transmission is not because it is inherently smaller. Instead, it's because power companies use higher AC voltage to reduce energy loss during long-distance transport. The use of high voltage means a lower current is required to transport the same power, reducing heat losses in the transmission lines. This makes AC transmission more efficient and cost-effective than transmitting the same power using low-voltage DC in many circumstances.
